number of transformer turns?

[synthetic]

Hello, I am trying to wind my own transformer for a power supply that I am making.

I am having trouble finding resources on the optimal number of windings. I know the ratio that I need to supply the correct voltage, but how do you decide what is the correct multiple of the ratio for a given frequency and load?

 

for example, 60hz 115vac to 24vac with a 3A load you would need a ratio of roughly 5/1. I have never seen a transformer with one winding on one side and five on the other. clearly that would not work well.In which case, how do you decide what to use, 25 - 5, 50 - 10 so on and so forth.

Thanks =D

[c4757p]

The simple answer is that it's a tradeoff - more turns per volt means higher inductance, which means a lower current draw when idle. It also means more wire, which means more resistance, which means more wasted heat per unit current drawn.

Typically the supplier of the transformer core will specify a number of turns per volt.

The right answer is that safe, proper transformer design is not simple, and you probably shouldn't be attempting it if you don't know the answer to that question already. Why do you need to wind one?

[jmoreland79]

for example, 60hz 115vac to 24vac with a 3A load you would need a ratio of roughly 5/1. I have never seen a transformer with one winding on one side and five on the other. clearly that would not work well.In which case, how do you decide what to use, 25 - 5, 50 - 10 so on and so forth.

Thanks =D

Well, you don't have to use the entire secondary winding, you can just tap off at the appropriate point that gives you the desired voltage.

But I'm with c4757; you probably shouldn't be trying to build your own transformer with mains level voltage on the primary if you really don't know how they work...

[rob77]

agree with the others, you shouldn't do that unless you really know what are you doing (but apparently you don't know; ))

if your budget is low and you need to keep the costs low, and you have a transformer with different secondary than you need, then replace the secondary winding only. while removing the secondary - count the turns and you will be able to calculate the turns per volt (the turns of the old secondary / voltage of the old secondary) and therefore calculate the correct number of turns for your new secondary.

but anyways - enameled copper wire is expensive, so you'll barely save anything by making your own transformer, so rather look after older cheap transformers - the 24V secondary is pretty common - you should have no issues to find one for cheap.

[rsjsouza]

If you google for "power transformer calculator" you can find several references on the web. I have used the one at this page, however it is in portuguese.

As others mentioned, be very careful when assembling and testing this - a good circuit breaker or fuse is always recommended when starting it. Also, double, triple and quadruple check your calculations, as any miscalculations will render your transformer completely unusable (it is frustrating to wind hundreds of turns just to see the voltage completely off or the wiring melt due to shorts and/or heat). That brings another issue to mind: always treat the enameled wire with care to avoid shorts. For the assembly itself, keep in mind the laminates tend to vibrate, therefore it is imperative to tie them together very strongly.

One excellent idea is to use a variac at the input. It is always good to start at very low voltages, which will reduce the energy delivered to the transformer and give you time to react if anything goes wrong.

[synthetic]

Oh, that was just an example. I am not going anywhere near mains. I am working with 24v ac from a properly designed mains transformer and i want to make mini transformers so i can have that go to 3.3, 5, 7, and 12v. I am not going to draw much current so I figured I could hack open an unused E frame transformer and wrap my own transformer with taps at all of the aforementioned voltages.

[G7PSK]

Have looked at this web site it has a lot of information on transformers.

http://ludens.cl/Electron/trafos/trafos.html

[Richard Crowley]

If you are winding your own primary, then YOU DEFINE what the "turns-per-volt" is.
As others have already observed, this is not a simple task and there are many trade-off decisions.

[XFDDesign]

Winding a transformer starts with your core material.

Unlike many RF transformers, it's not uncommon for power transformers to be composed of laminated steel plates. You'll first have to identify whether you have a ferrite material, or laminated steel plates. A given material will have a permeability which expresses a quantity of reluctance to a magnetic field, which tells you that for a specific number of turns, you'll get a particular amount of inductance.

Suppose you were using #77 core material for an E-core -500 set (http://www.amidoncorp.com/ea-77-500/) - The spec sheet for this core (http://www.amidoncorp.com/product_images/specifications/2-40.pdf) outlines 4470mH for 1000 turns (Al value).

Set that information aside for a moment.

The second thing you need to comprehend is that a transformer is a power device, not a voltage device. Voltage is a component of power. So is current. Recall that you have a resistance when you have V/I? Well transformers convert impedances in their power transformation operation. You specified that you want 24V with a 3A load; or with an 8 ohm load on the output, you want 24V supply.

Power out = 24 * 3 = 72 Watts

Your input voltage is 180V in amplitude for 120Vrms line supply. Figure your transformer is pretty efficient at 98% Work your current backwards:

Pin = 72/0.98 ~= 73.5W
73.5 = 180 * InputCurrent
InputCurrent = 0.41A
Resolve your effective input Z: 441Ohms.

Your turns ratio? sqrt(441/50) [pri/sec] = 7.42:1

Now, "how many turns?" - this is dictated by your primary side magnetics; the "L" value will be the magnetizing inductance of your transformer, and you will typically want the impedance to be at least 5x your specified value, preferably 10x. (As you get closer to 1x, your transformer starts becoming more of a parasite and your coupling goes down)

Zeffective_primary: 441 x 10 = 4410 Ohms.

Now you have a primary impedance target: 4.4kOhm, so solve the inductance:

4410 / (2 * 3.1415* 60Hz) = 11.7 H

If you go for the 5x factor: 5.8 H

So, how many turns? It goes back to the Al number for your particular core. If you were married to the -500 core, your Al = 4470mH/1000T

How many turns is 5.8H?

Num_turns = 1000 * sqrt( 5800mH/4470mH ) = 1140 Turns.

How many turns is 11.7H?

Num_turns = 1000 * sqrt( 11700mH/4470mH) = 1618  Turns

From here, you can resolve your secondary turns:

1140 T / 7.42 = 154 turns secondary
1618 T / 7.42 = 218 turns secondary

That's a lot of turns isn't it?

Well, the thing to understand that just as Capacitance is a function of Electric Field for volume of space (thickness and area), Inductance is a function of magnetic field for a given volume. Those really, really big transformers? They have an even larger Al value (often being a few 10s of H per 1kTurns).

So now you've done all the math work, and you have your basic numbers. Now comes the materials challenge: For the given number of turns you have calculated, for the given frequency of constraint, and the applied voltage available to your source, you have to calculate flux density for your core material. This is incidentally why laminated steel is popular for power transformers, as it has a very high flux capability before becoming saturated. Good manufacturers will provide the general equations for the calculation of the Bl quantity and give you a number to stay under. But since this is highly dependent on your source, you'll have to either get answers from your vendor, or calculate and measure where the core saturation hits.

Spec'ing a transformer design is actually an iterative process, like many things in Engineering.

Once you've completed your transformer, you should validate its basic operation with a function gen. Set it to 60 Hz, with a sine-wave output at max voltage. With the 50ohm source, your secondary impedance should be 0.9 Ohm. Fudge and use a 1Ohm resistor. If things are correct, for 10V input amplitude, you should resolve 1.347V output amplitude.

Once you have a basic verification of operation, you would be ready for testing it on a "live" system, however you would want to use an  isolation transformer, with an inline fuse, to cover your butt in case you did something wrong. Put an 8ohm load on your secondary output (rated to 100W). Your initial test would be best performed with a remote operation so that if anything explodes, you're not near it. If it passes these tests, you'll next want to have the system running, and take thermal measurements of the device to ensure it does not get exceedingly hot. If it's getting hot, something is wrong and it isn't a good design (likely the flux density is much higher than calculated and you'll have to turn another prototype to get the losses down).

Safety is key, especially with power systems. Don't blame me if you screw up, but there is the information needed.

Cheers.

[synthetic]

+1 to all that replied, between all of the links and the thorough walk-through from xfx I have more than enough material to chew on. Don't worry about safety, this is simply my preliminary inquisition. I always look before I leap.

I look forward to contributing to this forum in whatever ways I can in the future.
It seems like a great place.

Maxwell,